Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery includes: a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolytic solution. The negative electrode includes a negative electrode active material which contains silicon or a silicon alloy and a compound containing a first element, the electrolytic solution contains an imide salt which contains the first element and an imide anion, and the first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a lithium ion secondary battery. Priority is claimed on Japanese Patent Application No. 2021-088313 filed on May 26, 2021, the content of which is incorporated herein by reference.

Description of Related Art

Lithium ion secondary batteries are also widely utilized as a power source for mobile devices, such as mobile phones and notebook computers, and for hybrid cars.

The capacities of lithium ion secondary batteries mainly depend on the active material of electrodes. Graphite is generally utilized as a negative electrode active material, but a negative electrode active material with a higher capacity is required. Therefore, silicon (Si) and silicon oxide (SiO_(x)), which have a much larger theoretical capacity than the theoretical capacity (372 mAh/g) of graphite, are attracting attention.

Si and SiO_(x) have an accompanying large volume expansion during charging. When a negative electrode active material expands in volume, a solid electrolyte interface (SEI) film formed on the surface of the negative electrode active material is damaged and a new active surface is exposed. When this active surface comes into contact with an electrolytic solution, the electrolytic solution is decomposed and the cycle characteristics of the battery deteriorate.

The electrolytic solution is one of the factors that affect the cycle characteristics of the battery. For example, Patent Documents 1 to 3 describe electrolytic solutions containing a specific lithium salt and an ether compound.

[Patent Documents]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2009-176534

[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2014-110235

[Patent Document 3] Published Japanese Translation No. 2015-534254 of the PCT International Publication

SUMMARY OF THE INVENTION

Further improvement of cycle characteristics is required.

The present disclosure was made in view of the above problems, and an object of the present disclosure is to provide a lithium ion secondary battery having excellent cycle characteristics.

The following means are provided to achieve the above object.

(1) A lithium ion secondary battery according to a first aspect includes a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolytic solution. The negative electrode includes a negative electrode active material which contains silicon or a silicon alloy and a compound containing a first element. The electrolytic solution contains an imide salt which contains the first element and an imide anion. The first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.

(2) In the lithium ion secondary battery according to the above aspect, a molarity proportion of the imide salt containing the first element in the electrolytic solution may be 5% or more and 20% or less with respect to a molarity proportion of the lithium salt in the electrolytic solution.

(3) In the lithium ion secondary battery according to the above aspect, the compound containing the first element may be at least one selected from the group consisting of fluoride, oxide, silicide, silicic oxide, and phosphorus oxide.

(4) In the lithium ion secondary battery according to the above aspect, the negative electrode active material may include a core and a coating layer with which the core is coated. The coating layer may contain the compound containing the first element.

(5) In the lithium ion secondary battery according to the above aspect, the negative electrode active material may contain a silicon alloy, the silicon alloy may be represented by XnSi, X may be one or more elements which can be divalent or higher cations, and n may satisfy 0≤n≤0.5.

(6) In the lithium ion secondary battery according to the above aspect, a median diameter (D50) of the negative electrode active material may be 1 μm or more and 10 μm or less.

(7) In the lithium ion secondary battery according to the above aspect, a discharge specific capacity of the negative electrode active material may be 1000 mAh/g or more and 2500 mAh/g or less.

The lithium ion secondary battery according to the above aspect has excellent cycle characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment.

FIG. 2 is a schematic diagram of a negative electrode active material according to the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail below with reference to the drawings as appropriate. In the drawings used in the following description, an enlarged feature portion may be provided for convenience to make the feature easy to understand, and a dimensional ratio of each constituent element may be different from that of the actual constituent element. The materials, the dimensions, and the like exemplified in the following description are examples, the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist of the present invention.

“Lithium Ion Secondary Battery”

FIG. 1 is a schematic view of a lithium ion secondary battery according to a first embodiment. A lithium ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and a non-aqueous electrolytic solution (not shown). The periphery of the power generation element 40 is covered with the exterior body 50. The power generation element 40 is connected to the outside through a pair of terminals 60 and 62 which are connected. The non-aqueous electrolytic solution is accommodated in the exterior body 50.

(Power Generation Element)

The power generation element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

<Positive Electrode>

The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[Positive Electrode Current Collector]

The positive electrode current collector 22 is, for example, a conductive sheet material. The positive electrode current collector 22 is, for example, a thin metal sheet made of aluminum, copper, nickel, titanium, stainless steel, or the like. It is preferable that aluminum which has a light weight be utilized for the positive electrode current collector 22. An average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[Positive Electrode Active Material Layer]

The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive auxiliary agent and a binder if necessary.

A basis weight of the positive electrode active material layer 24 is, for example, 15 mg/cm² or more, and preferably 20 mg/cm² or more. The basis weight of the positive electrode active material layer 24 is, for example, preferably 35 mg/cm² or less. The basis weight means the mass of the positive electrode active material layer 24 supported on a surface of the positive electrode current collector 22 per unit area. When the basis weight is large, an amount of the positive electrode active material per unit area increases and the capacity of the battery increases. On the other hand, if the basis weight is too large, it becomes difficult for the electrolytic solution to impregnate into the positive electrode active material layer 24.

The positive electrode active material is an electrode active material capable of reversibly carrying out the absorption and desorption of lithium ions, the elimination and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂), lithium manganate (LiMnO₂), lithium manganese spinel (LiMn₂O₄), and a compound of the general expression: LiNi_(x)Co_(y)Mn_(z)M_(a)O₂ (in the general expression, x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, and 0≤a<1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV₂O₅), olivine type LiMPO₄ (where M indicates one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr or VO), lithium titanate (Li₄Ti₅O₁₂), and LiNi_(x)Co_(y)Al_(z)O₂ (0.9<x+y+z<1.1). The positive electrode active material may be an organic substance.

For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

The positive electrode active material may be a lithium-free material. The lithium-free material is, for example, FeF₃, a conjugated polymer containing an organic conductive substance, a chevrel phase compound, a transition metal chalcogenide, a vanadium oxide, a niobium oxide, or the like. As the lithium-free material, only one of these materials may be utilized or a combination of a plurality of materials may be utilized. When the positive electrode active material is a lithium-free material, for example, electric discharge is performed first. Lithium is inserted into the positive electrode active material through electric discharge. In addition, lithium may be chemically or electrochemically pre-doped into a lithium-free material for the positive electrode active material.

Conductive auxiliary agents enhance electron conductivity between positive electrode active materials. The conductive auxiliary agent may be, for example, a carbon powder, a carbon nanotube, a carbon material, a metal fine powder, a mixture of a carbon material and a metal fine powder, or a conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, and the like. The metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, or the like.

The binder binds the active material together. As the binder, a known binder can be utilized. The binder is, for example, a fluororesin. Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), and the like.

In addition to the above, examples of the binder include a vinylidene fluoride fluororubber such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidenefluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidenefluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber). Furthermore, the binder may be, for example, cellulose, a styrene/butadiene rubber, an ethylene/propylene rubber, a polyimide resin, a polyamide-imide resin, an acrylic resin, or the like.

Also, the binder may be a conductive polymer with electron conductivity or a conductive polymer with ion conductivity. Examples of the conductive polymer with electron conductivity include polyacetylene, polythiophene, polyaniline, and the like. Examples of the conductive polymer with ion conductivity include a composite substance of a polyether-based polymer compound and a lithium salt. Examples of the polyether polymer-based compound include a polyethylene oxide, a polypropylene oxide, and the like. Examples of the lithium salt include LiFSI, LiTFSI, LiBETI, LiClO₄, LiBF₄, LiPF₆, and the like.

A compositional proportion of the positive electrode active material in the positive electrode active material layer 24 is, for example, 80% or more and 98% or less in terms of mass ratio. Furthermore, a compositional proportion of the conductive auxiliary agent in the positive electrode active material layer 24 is, for example, 1.0% or more and 10% or less in terms of mass ratio. In addition, a compositional proportion of the binder in the positive electrode active material layer 24 is, for example, 1.0% or more and 10% or less in terms of mass ratio.

<Negative Electrode>

The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[Negative Electrode Current Collector]

The negative electrode current collector 32 is, for example, a conductive sheet material. As the negative electrode current collector 32, the same collector as that of the positive electrode current collector 22 can be utilized.

[Negative Electrode Active Material Layer]

The negative electrode active material layer 34 contains a negative electrode active material and a binder. Furthermore, the negative electrode active material layer 34 may contain a conductive auxiliary agent if necessary.

The negative electrode active material includes silicon or a silicon alloy and a compound containing a first element. The silicon or the silicon alloy may be crystalline or amorphous. The silicon alloy is represented by XnSi. X is a cation. X is, for example, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Ti, Na, or K. It is preferable that X be one or more elements that can be divalent or higher cations. Elements which can be divalent or higher cations are, for example, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, and Ti. n satisfies 0≤n≤0.5.

The compound containing the first element is, for example, one or more selected from the group consisting of fluoride, oxide, silicide, and silicon oxide. The first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn. The first element is not limited to one type of element and may be of a plurality of types of element groups. For example, when the first element is Mg, the compounds containing the first element are, for example, MgF₂, MgO, Mg₂Si, Li_(x)Mg_(y)Si, and Mg₂SiO₄.

FIG. 2 is a schematic view of a negative electrode active material 35 according to the first embodiment. The negative electrode active material 35 may include a core 36 and a coating layer 37. At least a part of the core 36 is covered with the coating layer 37. The entire surface of the core 36 may be covered with the coating layer 37. The coating layer 37 makes an insertion/desorption reaction of Li uniform and minimizes deterioration of the negative electrode active material 35.

The core 36 is silicon or a silicon alloy. The coating layer 37 contains, for example, silicon or a silicon alloy and a compound containing the first element. The coating layer 37 may contain a carbon material, a metal fine powder, a mixture of a carbon material and a metal fine powder, a conductive oxide, an inorganic compound, and the like. The carbon material is, for example, a carbon powder, a carbon nanotube, or the like. The metal fine powder is, for example, copper, cobalt, iron, or the like. The inorganic material is, for example, ceramics, oxides, carbides, nitrides, borides, and the like.

The coating layer 37 may include a first layer 37A and a second layer 37B. The second layer 37B is the outermost surface layer of the coating layer 37. The second layer 37B may contain a reaction product of the first element and the material contained in the electrolytic solution. For example, when the first element is Mg, the reaction product of the first element and the material contained in the electrolytic solution is, for example, Mg₃(PO₄)₂.

A median diameter (D50) of the negative electrode active material is, for example, 1 μm or more and 10 μm or less. When a particle size of the negative electrode active material decreases, a specific surface area of the negative electrode active material increases. When the specific surface area of the negative electrode active material increases, a frequency of contact between the electrolytic solution and the negative electrode active material increases and the electrolytic solution is easily decomposed. On the other hand, when the particle size of the negative electrode active material increases, a local low potential or high potential locations are less likely to occur on the surface of the negative electrode active material. Non-uniformity of the potential can cause redox decomposition of the electrolytic solution. When the median diameter of the negative electrode active material is within the above range, the cycle characteristics of the lithium ion secondary battery 100 are improved.

The discharge specific capacity of the negative electrode active material is, for example, 1000 mAh/g or more and 2500 mAh/g or less. The discharge specific capacity of the negative electrode active material can be adjusted by changing a thickness of the positive electrode active material layer 24 of the positive electrode 20. When the discharge specific capacity of the negative electrode active material increases, the negative electrode has a low potential. In addition, when the discharge specific capacity of the negative electrode active material decreases, the negative electrode has a high potential. A high or low potential of the negative electrode can cause deterioration of cycle characteristics. When the negative electrode potential increases, the amount of lithium that can be trapped increases. Here, the electrolytic solution and the coating layer 37 are likely to be oxidatively decomposed. When the negative electrode potential decreases, the expansion and contraction of the negative electrode active material 35 can be minimized. Here, the electrolytic solution and the coating layer 37 are easily reduced and decomposed. When a discharge specific capacity of the negative electrode active material is within the above range, the capacity is high and the cycle characteristics are excellent.

The conductive auxiliary agent enhances electronic conductivity between the negative electrode active materials. As the conductive auxiliary agent, the same material as that of the positive electrode 20 can be utilized.

The binder binds the negative electrode active materials together and also binds the negative electrode active material and the negative electrode current collector 32. As the binder, the same binder as that used for the positive electrode 20 can be utilized.

The contents of the negative electrode active material, the conductive auxiliary agent, and the binder in the negative electrode active material layer 34 are not particularly limited. The compositional proportion of the negative electrode active material in the negative electrode active material layer 34 is, for example, 70% or more and 100% or less in terms of mass ratio. Furthermore, the compositional proportion of the conductive auxiliary agent in the negative electrode active material layer 34 is, for example, 0% or more and 10% or less in terms of mass ratio. The compositional proportion of the binder in the negative electrode active material layer 34 is, for example, 0% or more and 20% or less in terms of mass ratio.

<Separator>

The separator 10 is disposed between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in a plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or a laminated body formed of a polyolefin film. The separator 10 may be a stretching film made of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may be an inorganic coated separator. The inorganic coated separator is obtained by coating a surface of the above film with a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica. The inorganic coated separator has excellent heat resistance and minimizes the precipitation of transition metals eluted from the positive electrode on a surface of the negative electrode.

<Electrolytic Solution>

The electrolytic solution is sealed in the exterior body 50 and impregnated into the power generation element 40. The non-aqueous electrolytic solution has, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in a non-aqueous solvent. The electrolytic solution contains, for example, a solvent, an electrolyte, and an imide salt. The imide salt may be of one kind or two or more kinds.

The solvent is not particularly limited as long as it is a solvent generally used for lithium ion secondary batteries. The solvent includes, for example, any of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain a mixture of these in any proportions. The cyclic carbonate compound is, for example, ethylene carbonate (EC), propylene carbonate (PC), or the like. The chain carbonate compound is, for example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or the like. The cyclic ester compound is, for example, y-butyrolactone or the like. The chain ester compound is, for example, propyl propionate, ethyl propionate, ethyl acetate, or the like.

The solvent may include a fluorinated organic solvent. The fluorinated organic solvent forms a good coating film on a surface of the negative electrode active material. When a decomposition product of the fluorinated organic solvent and the first element contained in the electrolytic solution are contained in the coating film, Li ions are easily transported in the coating film. As a result, an increase in the resistance of the lithium ion secondary battery 100 is minimized.

The electrolyte is, for example, a lithium salt. Examples of the electrolyte include LiPF₆, LiClO₄, LiBF₄, LiCF₃SO₃, LiCF₃CF₂SO₃, LiC(CF₃SO₂)₃, LiN(CF₃SO₂)₂, LiN(CF₃CF₂SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiN(CF₃CF₂CO)₂, LiBOB, and the like. One type of lithium salt may be used alone or a combination of two or more types of lithium salts may be used. From the viewpoint of a degree of ionization, it is preferable that the electrolyte contain LiPF₆.

The imide salt contains a first element and an imide anion. The first element is as described above. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 are, for example, the same element. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 may be different elements. The imide salt is not limited to one type and a plurality of types may be contained in the electrolytic solution.

Examples of the imide anion include (SO₂F)₂N⁻(FSI⁻: bis(fluorosulfonyl)imide anion), (SO₂CF₃)₂N⁻(TFSI⁻: bis(trifluoromethanesulfonyl)imide anion), (SO₂C₂F₅)₂N⁻(BETI⁻: bis(pentafluoroethanesulfonyl)imide anion), (SO₂F)(SO₂CF₃)N⁻, and (SO₂CF₃)(SO₂C₂F₅)N⁻. One type of imide anion may be used alone or a combination of two or more types of imide anions may be used.

A molarity proportion of the imide salt containing the first element in the electrolytic solution is, for example, 5% or more and 20% or less with respect to a molarity proportion of the lithium salt in the electrolytic solution. If sufficient imide salt of the first element is contained in the electrolytic solution, decomposition of the electrolytic solution is prevented during charging and discharging of the lithium ion secondary battery 100. Furthermore, if excess imide salt of the first element is contained in the electrolytic solution, the viscosity of the electrolytic solution increases and the ability of the electrolytic solution to become impregnated into the positive electrode 20 and the negative electrode 30 decreases.

The molarity proportion of the first element and lithium in the electrolytic solution can be measured through, for example, gas chromatography, inductively coupled plasma (ICP) emission spectrometry, ICP mass analysis, and the like.

<Exterior Body>

The exterior body 50 seals the power generation element 40 and the non-aqueous electrolytic solution therein. The exterior body 50 suppresses leakage of the non-aqueous electrolytic solution to the outside and introduction of water and the like into the inside of the lithium ion secondary battery 100 from the outside.

The exterior body 50 has, for example, as shown in FIG. 1 , a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film obtained by coating the metal foil 52 with a polymer film (resin layer 54) from both sides.

As the metal foil 52, for example, an aluminum foil can be used. A polymer film of polypropylene or the like can be used for the resin layer 54. A material constituting the resin layer 54 may be different between the inside and the outside. For example, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide (PA), or the like can be used as an outer material and polyethylene (PE), polypropylene (PP), or the like can be used as a material of an inner polymer film.

<Terminal>

The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection with the outside. The terminals 60 and 62 are formed of a conductive material such as aluminum, nickel, and copper. A connection method may be welding or screwing. It is preferable to protect the terminals 60 and 62 with an insulating tape to prevent a short circuit.

“Method for Producing Lithium Ion Secondary Battery”

The lithium ion secondary battery 100 is prepared by preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolytic solution, and the exterior body 50 and assembling them. An example of a method for producing the lithium ion secondary battery 100 will be described below.

The negative electrode 30 is prepared through, for example, a compounding step, a slurry manufacturing step, an electrode coating step, a drying step, and a rolling step in this order.

The compounding step is a step of mixing silicon or a silicon alloy with a compound containing the first element while applying a shearing force. When the silicon or the silicon alloy is mixed while applying a shearing force to the extent that the material does not deteriorate, the surface of the silicon or the silicon alloy is coated with the compound containing the first element. Furthermore, the particle size of the negative electrode active material can be adjusted by the degree of mixing. In addition, the prepared negative electrode active material may be sieved to make the particle size uniform.

The slurry manufacturing step is a step of mixing the combined negative electrode active material, the binder, and the solvent as a compound to prepare a slurry. In the slurry manufacturing step, a conductive auxiliary agent may be added if necessary. The solvent is, for example, water, N-methyl-2-pyrrolidone, and the like. The compositional proportions of the negative electrode active material, the conductive material, and the binder is preferably 70 wt % to 100 wt %:0 wt % to 10 wt %:0 wt % to 20 wt % in terms of mass ratio. These mass ratios are adjusted to be 100 wt % as a whole.

The electrode coating step is a step of coating a surface of the negative electrode current collector 32 with the slurry. A slurry coating method is not particularly limited. For example, a slit die coat method and a doctor blade method can be utilized as the slurry coating method.

The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere of 80° C. to 150° C. When the slurry dries, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

The rolling step is performed if necessary. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust a density of the negative electrode active material layer 34. The rolling step is performed using, for example, a roll press device or the like.

The positive electrode 20 can be prepared by the same procedure as that of the negative electrode 30 except that the compounding step is not performed. As the separator 10 and the exterior body 50, commercially available separator and exterior body can be utilized.

The electrolytic solution can be prepared, for example, by adding the imide salt containing the first element and the imide anion to the mixture of the lithium salt and the solvent and mixing them.

Subsequently, the power generation element 40 is prepared by laminating the separator 10 and the prepared positive electrode 20 and negative electrode 30 so that the separator 10 is located between the prepared positive electrode 20 and negative electrode 30. When the power generation element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound using one end side of the positive electrode 20, the negative electrode 30, and the separator 10 as an axis.

Finally, the power generation element 40 is sealed in the exterior body 50. The non-aqueous electrolytic solution is injected into the exterior body 50. The non-aqueous electrolytic solution is impregnated into the power generation element 40 by injecting the non-aqueous electrolytic solution and then reducing the pressure, the heating, or the like. When the exterior body 50 is sealed by applying heat or the like, the lithium ion secondary battery 100 can be obtained. The power generation element 40 may be impregnated into the electrolytic solution instead of injecting the electrolytic solution into the exterior body 50.

The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics. When the active surface generated through the volume expansion of the negative electrode active material comes into contact with the solvent, the solvent may be decomposed and the cycle characteristics of the lithium ion secondary battery may be deteriorated in some cases. On the other hand, the lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the contact between the active surface and the solvent is prevented.

The compound containing the first element is polarized in the negative electrode active material during a charge/discharge reaction. The polarized compound attracts lithium ions. When lithium ions are attracted to the vicinity of the negative electrode active material, the solvent having an unshared electron pair moves away from the negative electrode active material relatively as compared with the electrolyte and the imide salt containing the first element. The probability of contact between the active surface and the solvent is lower than the probability of contact between the active surface and the electrolyte (lithium salt) or the imide salt. The active surface reacts with an electrolyte (lithium salt) or an imide salt instead of a solvent to reshape the SEI film. That is to say, since the decomposition of the solvent by the reaction with the active surface is prevented, the lithium ion secondary battery 100 has excellent cycle characteristics.

Also, if the electrolytic solution contains an imide salt containing the first element, the first element is incorporated into the negative electrode active material during a charge/discharge reaction of the lithium ion secondary battery 100. Even when the negative electrode active material has an active surface due to expansion and contraction, the first element is supplied from the electrolytic solution so that the first element functions as an anchor for the negative electrode active material. As a result, the strength of the negative electrode active material is increased, and the decay of the negative electrode active material can be prevented.

Furthermore, a reaction between the active surface and the electrolyte (lithium salt) or the imide salt described above occurs on the surface of the negative electrode active material. For this reason, when the first element is contained in the coating layer 37 of the negative electrode active material, the effect of preventing the deterioration of the cycle characteristics of the lithium ion secondary battery 100 is high.

Although the embodiments of the present invention have been described in detail above with reference to the drawings, each constitution and a combination thereof in each embodiment are examples and the constitutions can be added, omitted, replaced, and other changes are possible without departing from the gist of the present invention.

EXAMPLES Example 1

One surface of an aluminum foil having a thickness of 15 μm was coated with a positive electrode slurry. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive auxiliary agent, a binder, and a solvent.

Li_(x)CoO₂ was used as the positive electrode active material. Acetylene black was as the conductive auxiliary agent. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. The positive electrode slurry was prepared by mixing 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive auxiliary agent, 2 parts by mass of the binder, and 70 parts by mass of the solvent. An amount of a positive electrode active material supported on a dried positive electrode active material layer was 25 mg/cm². The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer. A positive electrode was prepared by pressing the positive electrode active material layer through a roll press.

A negative electrode active material was prepared by combining crystallized silicon, magnesium oxide, and LiF as a compound using Mechanofusion manufactured by Hosokawa Micron tilted at 10 degrees. A rotation speed of the device at the time of performing compounding was set to 2500 rpm. As the crystallized silicon, crystallized silicon having a median diameter (D50) of 3 μm was utilized. Magnesium oxide was an oxide containing the first element. A median diameter of the negative electrode active material which has been subjected to the compounding treatment was 3 μm. A coating layer containing Mg was formed on a surface of the negative electrode active material.

Subsequently, one surface of a copper foil having a thickness of 10 μm was coated with a negative electrode slurry. A negative electrode slurry was prepared by mixing a negative electrode active material, a conductive auxiliary agent, a binder, and a solvent. A negative electrode active material had been subjected to the above compounding treatment. Carbon black was used as the conductive auxiliary agent. A polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. A negative electrode slurry was prepared by mixing 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive auxiliary agent, and 5 parts by mass of the binder with N-methyl-2-pyrrolidone. An amount of a negative electrode active material supported on a dried negative electrode active material layer was 2.5 mg/cm². A negative electrode active material layer was prepared by removing the solvent from a negative electrode slurry in a drying furnace. The negative electrode active material layer was pressed through a roll press and then heat-fired at 300° C. or higher for 5 hours in a nitrogen atmosphere.

Subsequently, the solvent was prepared by mixing fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) so that a volume ratio was fluoroethylene carbonate (FEC):ethylene carbonate (EC):propylene carbonate (PC):diethyl carbonate (DEC)=5:5:20:70. LiPF₆ was dissolved in this mixed solvent to have a concentration of 1 mol/L. After that, with respect to this solution, the electrolytic solution was prepared by adding Mg(TFSI)₂ as an imide salt to the electrolytic solution. The imide salt concentration of the first element with respect to the Li salt concentration was adjusted to be 10% (0.1 mol/L).

(Preparation of Lithium Ion Secondary Battery for Evaluation)

A laminated body was obtained by laminating the prepared negative electrode and positive electrode via the separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer faced each other. A nickel negative electrode lead was attached to a negative electrode of the laminated body. An aluminum positive electrode lead was attached to a positive electrode of the laminated body. The positive electrode lead and the negative electrode lead were welded through an ultrasonic welder. This laminated body was inserted into the exterior body formed of an aluminum laminated film and heat-sealed except for one peripheral portion to form a closed portion. Furthermore, finally, after injecting the electrolytic solution into the exterior body, the remaining one place was sealed with a heat seal while reducing the pressure with a vacuum sealer to prepare a lithium ion secondary battery. Two lithium ion secondary batteries such as one lithium ion secondary battery for analyzing the composition of the electrolytic solution and the other lithium ion secondary battery for evaluating the charge/discharge characteristics were prepared.

Also, an electrolytic solution was collected from a lithium ion secondary battery for electrolyte composition analysis. Subsequently, a composition of the collected electrolytic solution was analyzed using ICP emission. As a result, it was confirmed that the content of Mg in the electrolytic solution was the same as the content at the time of preparing the electrolytic solution.

(Measurement of Capacity Retention Rate after 300 cycles)

The cycle characteristics of the lithium ion secondary battery for evaluation were measured. The cycle characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Corporation).

Charge was performed until the battery voltage reached 4.4 V with constant current charging with a charging rate of 1.0 C (current value in which charging was completed in 1 hour when constant current charging was performed at 25° C.) and discharging was performed with a constant current discharge at a discharge rate of 1.0 C until the battery voltage reached 3.0 V. The discharge capacity after the end of charging and discharging was detected and a battery capacity Q₁ before the cycle test was obtained.

The battery for which the battery capacity Q₁ was obtained above was charged through constant current charging at a charge rate of 1.0 C until the battery voltage reached 4.4 V and discharged through constant current discharging at a discharge rate of 1.0 C until the battery voltage reached 3.0 V using a secondary battery charge/discharge test device again. The charge/discharge was counted as one cycle and 300 cycles of charge/discharge were performed. After that, the discharge capacity after the end of 300 cycles of charging and discharging was detected and a battery capacity Q₂ after 300 cycles was obtained.

Based on the capacities Q₁ and Q₂ obtained above, a capacity retention rate E after 300 cycles was determined. The capacity retention rate E was obtained by E=Q₂/Q₁×100. The capacity retention rate E of Example 1 was 77%.

Examples 2 to 9

Examples 2 to 9 were different from Example 1 in that at least one of an imide salt added to an electrolytic solution and a first element contained in a negative electrode active material was changed. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

Examples 10 to 14

Examples 10 to 14 were different from Example 1 in that a Li salt concentration and an imide salt concentration of a first element in an electrolytic solution are changed. In each of Examples 10 to 14, the imide salt concentration of the first element was adjusted to 10% with respect to the Li salt concentration in the electrolytic solution. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

Examples 15 to 20

Examples 15 to 20 were different from Example 1 in that a Li salt concentration in an electrolytic solution is fixed and an imide salt concentration of a first element is changed. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

Examples 21 to 24

Examples 21 to 24 were different from Example 1 in that an imide salt added to an electrolytic solution and a first element contained in a negative electrode active material were changed. In Examples 21 to 24, the first element contained in the imide salt and the first element contained in the negative electrode active material were both two or more kinds. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

Examples 25 to 38

Examples 25 to 38 were different from Example 1 in that a silicon alloy is used instead of crystallized silicon at the time of preparing a negative electrode active material. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

Examples 39 to 44

Examples 39 to 44 were different from Example 1 in that a particle size of a negative electrode active material was changed. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

Examples 45 to 49

Examples 45 to 49 were different from Example 1 in that a discharge specific capacity of a negative electrode active material was changed. The discharge specific capacity of the negative electrode active material was adjusted by changing a thickness of a positive electrode active material layer. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

Comparative Example 1

Comparative Example 1 was different from Example 1 in that magnesium oxide (compound containing first element) was not added when a negative electrode active material was prepared and an imide salt was not added to an electrolytic solution. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

Comparative Example 2

Comparative Example 2 was different from Example 1 in that an imide salt was not added to an electrolytic solution. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

Comparative Example 3

Comparative Example 3 was different from Example 1 in that magnesium oxide (compound containing first element) was not added when a negative electrode active material was prepared. As for other conditions, a capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

In Examples 1 to 49, the capacity retention rates were higher than those in Comparative Examples 1 to 3. That is to say, the lithium ion secondary batteries according to Examples 1 to 49 in which the negative electrode active material contained the compound contained the first element and the electrolytic solution contained the imide salt of the first element had excellent cycle characteristics.

TABLE 1 Cycle Negative electrode active material Electrolytic solution characteristic Discharge Imide salt Capacity Particle specific Li salt Imide salt concentration/Li retention Core size First capacity concentration concentration salt concentration rate E material (μm) element (mAh/g) Imide salt (mol/L) (mol/L) (%) (%) Example 1 Si 3 Mg 1520 Mg(TFSI)₂ 1.0 0.1 10 77 Example 2 Si 3 Mg 1520 Mg(FSI)₂ 1.0 0.1 10 76 Example 3 Si 3 Mg 1502 Mg(BETI)₂ 1.0 0.1 10 74 Example 4 Si 3 Ca 1530 Ca(TFSI)₂ 1.0 0.1 10 71 Example 5 Si 3 Cs 1500 Cs(TFSI)₂ 1.0 0.1 10 72 Example 6 Si 3 Al 1580 Al(TFSI)₃ 1.0 0.1 10 73 Example 7 Si 3 Zn 1500 Zn(TFSI)₂ 1.0 0.1 10 72 Example 8 Si 3 Na 1500 NaFSI 1.0 0.1 10 60 Example 9 Si 3 K 1500 KFSI 1.0 0.1 10 63 Example 10 Si 3 Mg 1490 Mg(TFSI)₂ 0.8 0.08 10 72 Example 11 Si 3 Mg 1520 Mg(TFSI)₂ 1.1 0.11 10 76 Example 12 Si 3 Mg 1520 Mg(TFSI)₂ 2.0 0.20 10 74 Example 13 Si 3 Mg 1480 Mg(TFSI)₂ 3.8 0.38 10 77 Example 14 Si 3 Mg 1430 Mg(TFSI)₂ 4.5 0.45 10 79 Example 15 Si 3 Mg 1580 Mg(TFSI)₂ 1.0 0.01 1 59 Example 16 Si 3 Mg 1550 Mg(TFSI)₂ 1.0 0.03 3 60 Example 17 Si 3 Mg 1540 Mg(TFSI)₂ 1.0 0.06 6 71 Example 18 Si 3 Mg 1540 Mg(TFSI)₂ 1.0 0.18 18 76 Example 19 Si 3 Mg 1490 Mg(TFSI)₂ 1.0 0.24 25 66 Example 20 Si 3 Mg 1450 Mg(TFSI)₂ 1.0 0.3 30 62 Example 21 Si 3 Mg 1530 Mg(TFSI)₂ 1.0 0.1 10 78 Ca Ca(TFSI)₂ Example 22 Si 3 Mg 1580 Mg(TFSI)₂ 1.0 0.1 10 80 Al Al(TFSI)₃ Example 23 Si 3 Mg 1500 Mg(TFSI)₂ 1.0 0.1 10 80 Zn Zn(TFSI)₂ Example 24 Si 3 Mg 1550 Mg(TFSI)₂ 1.0 0.1 10 81 Ca Ca(TFSI)₂ Al Al(TFSI)₃

TABLE 2 Cycle Negative electrode active material Electrolytic solution characteristic Discharge Imide salt Capacity Particle specific Li salt Imide salt concentration/Li retention Core size First capacity concentration concentration salt concentration rate E material (μm) element (mAh/g) Imide salt (mol/L) (mol/L) (%) (%) Example 25 Mg_(0.2)Si 3 Mg 1440 Mg(TFSI)₂ 1.0 0.1 10 75 Example 26 Al_(0.2)Si 3 Mg 1400 Mg(TFSI)₂ 1.0 0.1 10 73 Example 27 Zn_(0.2)Si 3 Mg 1320 Mg(TFSI)₂ 1.0 0.1 10 71 Example 28 Sn_(0.2)Si 3 Mg 1350 Mg(TFSI)₂ 1.0 0.1 10 72 Example 29 Ca_(0.2)Si 3 Mg 1290 Mg(TFSI)₂ 1.0 0.1 10 70 Example 30 V_(0.2)Si 3 Mg 1480 Mg(TFSI)₂ 1.0 0.1 10 76 Example 31 Cr_(0.2)Si 3 Mg 1450 Mg(TFSI)₂ 1.0 0.1 10 75 Example 32 Mn_(0.2)Si 3 Mg 1460 Mg(TFSI)₂ 1.0 0.1 10 72 Example 33 Fe_(0.2)Si 3 Mg 1520 Mg(TFSI)₂ 1.0 0.1 10 74 Example 34 Co_(0.2)Si 3 Mg 1520 Mg(TFSI)₂ 1.0 0.1 10 75 Example 35 Ni_(0.2)Si 3 Mg 1510 Mg(TFSI)₂ 1.0 0.1 10 71 Example 36 Ti_(0.2)Si 3 Mg 1520 Mg(TFSI)₂ 1.0 0.1 10 75 Example 37 Na_(0.2)Si 3 Mg 1400 Mg(TFSI)₂ 1.0 0.1 10 62 Example 38 K_(0.2)Si 3 Mg 1400 Mg(TFSI)₂ 1.0 0.1 10 61 Example 39 Si 0.2 Mg 1310 Mg(TFSI)₂ 1.0 0.1 10 60 Example 40 Si 0.8 Mg 1440 Mg(TFSI)₂ 1.0 0.1 10 62 Example 41 Si 1.2 Mg 1470 Mg(TFSI)₂ 1.0 0.1 10 73 Example 42 Si 4.6 Mg 1500 Mg(TFSI)₂ 1.0 0.1 10 76 Example 43 Si 8 Mg 1520 Mg(TFSI)₂ 1.0 0.1 10 75 Example 44 Si 12 Mg 1580 Mg(TFSI)₂ 1.0 0.1 10 66 Example 45 Si 3 Mg 880 Mg(TFSI)₂ 1.0 0.1 10 66 Example 46 Si 3 Mg 1260 Mg(TFSI)₂ 1.0 0.1 10 80 Example 47 Si 3 Mg 2360 Mg(TFSI)₂ 1.0 0.1 10 74 Example 48 Si 3 Mg 2810 Mg(TFSI)₂ 1.0 0.1 10 69 Example 49 Si 3 Mg 3030 Mg(TFSI)₂ 1.0 0.1 10 64 Comparative Si 3 — 1500 — 1.0 — — 50 Example 1 Comparative Si 3 Mg 1580 — 1.0 — — 57 Example 2 Comparative Si 3 — 1580 Mg(TFSI)₂ 1.0 0.1 10 57 Example 3

EXPLANATION OF REFERENCES

-   10 Separator -   20 Positive electrode -   22 Positive electrode current collector -   24 The positive electrode active material layer -   30 Negative electrode -   32 Negative electrode current collector -   34 Negative electrode active material layer -   35 Negative electrode active material -   36 Core -   37 Coating layer -   37A First layer -   37B Second layer -   40 Power generation element -   50 Exterior body -   52 Metal foil -   54 Resin layer -   60, 62 Terminal -   100 Lithium ion secondary battery 

What is claimed is:
 1. A lithium ion secondary battery comprising: a positive electrode; a negative electrode; a separator located between the positive electrode and the negative electrode; and an electrolytic solution, wherein the negative electrode includes a negative electrode active material which contains silicon or a silicon alloy and a compound containing a first element, the electrolytic solution contains an imide salt which contains the first element and an imide anion, and the first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.
 2. The lithium ion secondary battery according to claim 1, wherein a molarity proportion of the imide salt containing the first element in the electrolytic solution is 5% or more and 20% or less with respect to a molarity proportion of the lithium salt in the electrolytic solution.
 3. The lithium ion secondary battery according to claim 1, wherein the compound containing the first element is at least one selected from the group consisting of fluoride, oxide, silicide, silicic oxide, and phosphorus oxide.
 4. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material includes a core and a coating layer with which the core is coated, and the coating layer contains the compound containing the first element.
 5. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material contains a silicon alloy, the silicon alloy is represented by XnSi, X is one or more elements which can be divalent or higher cations, and n satisfies 0≤n≤0.5.
 6. The lithium ion secondary battery according to claim 1, wherein a median diameter (D50) of the negative electrode active material is 1 μm or more and 10 μm or less.
 7. The lithium ion secondary battery according to claim 1, wherein a discharge specific capacity of the negative electrode active material is 1000 mAh/g or more and 2500 mAh/g or less. 